Ubuntu Manpage: xargs - build and execute command lines from standard input

Provided by: findutils_4.6.0+git+20160126-2_amd64

NAME

       xargs - build and execute command lines from standard input

SYNOPSIS

       xargs  [-0prtx]  [-E  eof-str]  [-e[eof-str]]  [--eof[=eof-str]]  [--null]  [-d delimiter]
       [--delimiter  delimiter]  [-I  replace-str]  [-i[replace-str]]   [--replace[=replace-str]]
       [-l[max-lines]]  [-L  max-lines]  [--max-lines[=max-lines]] [-n max-args] [--max-args=max-
       args]  [-s  max-chars]  [--max-chars=max-chars]  [-P  max-procs]   [--max-procs=max-procs]
       [--process-slot-var=name]   [--interactive]   [--verbose]   [--exit]   [--no-run-if-empty]
       [--arg-file=file] [--show-limits] [--version] [--help] [command [initial-arguments]]

DESCRIPTION

This manual page documents the GNU version of xargs. xargs reads items from the standard
input, delimited by blanks (which can be protected with double or single quotes or a
backslash) or newlines, and executes the command (default is /bin/echo) one or more times
with any initial-arguments followed by items read from standard input. Blank lines on the
standard input are ignored.

The command line for command is built up until it reaches a system-defined limit (unless
the -n and -L options are used). The specified command will be invoked as many times as
necessary to use up the list of input items. In general, there will be many fewer
invocations of command than there were items in the input. This will normally have
significant performance benefits. Some commands can usefully be executed in parallel too;
see the -P option.

Because Unix filenames can contain blanks and newlines, this default behaviour is often
problematic; filenames containing blanks and/or newlines are incorrectly processed by
xargs. In these situations it is better to use the -0 option, which prevents such
problems. When using this option you will need to ensure that the program which produces
the input for xargs also uses a null character as a separator. If that program is GNU
find for example, the -print0 option does this for you.

If any invocation of the command exits with a status of 255, xargs will stop immediately
without reading any further input. An error message is issued on stderr when this
happens.

OPTIONS

-0, --null
Input items are terminated by a null character instead of by whitespace, and the
quotes and backslash are not special (every character is taken literally).
Disables the end of file string, which is treated like any other argument. Useful
when input items might contain white space, quote marks, or backslashes. The GNU
find -print0 option produces input suitable for this mode.

-a file, --arg-file=file
Read items from file instead of standard input. If you use this option, stdin
remains unchanged when commands are run. Otherwise, stdin is redirected from
/dev/null.

--delimiter=delim, -d delim
Input items are terminated by the specified character. The specified delimiter may
be a single character, a C-style character escape such as \n, or an octal or
hexadecimal escape code. Octal and hexadecimal escape codes are understood as for
the printf command. Multibyte characters are not supported. When processing the
input, quotes and backslash are not special; every character in the input is taken
literally. The -d option disables any end-of-file string, which is treated like
any other argument. You can use this option when the input consists of simply
newline-separated items, although it is almost always better to design your program
to use --null where this is possible.

-E eof-str
Set the end of file string to eof-str. If the end of file string occurs as a line
of input, the rest of the input is ignored. If neither -E nor -e is used, no end
of file string is used.

-e[eof-str], --eof[=eof-str]
This option is a synonym for the -E option. Use -E instead, because it is POSIX
compliant while this option is not. If eof-str is omitted, there is no end of file
string. If neither -E nor -e is used, no end of file string is used.

-I replace-str
Replace occurrences of replace-str in the initial-arguments with names read from
standard input. Also, unquoted blanks do not terminate input items; instead the
separator is the newline character. Implies -x and -L 1.

-i[replace-str], --replace[=replace-str]
This option is a synonym for -Ireplace-str if replace-str is specified. If the
replace-str argument is missing, the effect is the same as -I{}. This option is
deprecated; use -I instead.

-L max-lines
Use at most max-lines nonblank input lines per command line. Trailing blanks cause
an input line to be logically continued on the next input line. Implies -x.

-l[max-lines], --max-lines[=max-lines]
Synonym for the -L option. Unlike -L, the max-lines argument is optional. If max-
lines is not specified, it defaults to one. The -l option is deprecated since the
POSIX standard specifies -L instead.

-n max-args, --max-args=max-args
Use at most max-args arguments per command line. Fewer than max-args arguments
will be used if the size (see the -s option) is exceeded, unless the -x option is
given, in which case xargs will exit.

-P max-procs, --max-procs=max-procs
Run up to max-procs processes at a time; the default is 1. If max-procs is 0,
xargs will run as many processes as possible at a time. Use the -n option or the
-L option with -P; otherwise chances are that only one exec will be done. While
xargs is running, you can send its process a SIGUSR1 signal to increase the number
of commands to run simultaneously, or a SIGUSR2 to decrease the number. You cannot
increase it above an implementation-defined limit (which is shown with --show-
limits). You cannot decrease it below 1. xargs never terminates its commands;
when asked to decrease, it merely waits for more than one existing command to
terminate before starting another.

Please note that it is up to the called processes to properly manage parallel
access to shared resources. For example, if more than one of them tries to print
to stdout, the ouptut will be produced in an indeterminate order (and very likely
mixed up) unless the processes collaborate in some way to prevent this. Using some
kind of locking scheme is one way to prevent such problems. In general, using a
locking scheme will help ensure correct output but reduce performance. If you
don't want to tolerate the performance difference, simply arrange for each process
to produce a separate output file (or otherwise use separate resources).

-p, --interactive
Prompt the user about whether to run each command line and read a line from the
terminal. Only run the command line if the response starts with `y' or `Y'.
Implies -t.

--process-slot-var=name
Set the environment variable name to a unique value in each running child process.
Values are reused once child processes exit. This can be used in a rudimentary
load distribution scheme, for example.

-r, --no-run-if-empty
If the standard input does not contain any nonblanks, do not run the command.
Normally, the command is run once even if there is no input. This option is a GNU
extension.

-s max-chars, --max-chars=max-chars
Use at most max-chars characters per command line, including the command and
initial-arguments and the terminating nulls at the ends of the argument strings.
The largest allowed value is system-dependent, and is calculated as the argument
length limit for exec, less the size of your environment, less 2048 bytes of
headroom. If this value is more than 128KiB, 128Kib is used as the default value;
otherwise, the default value is the maximum. 1KiB is 1024 bytes. xargs
automatically adapts to tighter constraints.

--show-limits
Display the limits on the command-line length which are imposed by the operating
system, xargs' choice of buffer size and the -s option. Pipe the input from
/dev/null (and perhaps specify --no-run-if-empty) if you don't want xargs to do
anything.

-t, --verbose
Print the command line on the standard error output before executing it.

-x, --exit
Exit if the size (see the -s option) is exceeded.

--help Print a summary of the options to xargs and exit.

--version
Print the version number of xargs and exit.

EXAMPLES

       find /tmp -name core -type f -print | xargs /bin/rm -f

       Find files named core in or below the directory /tmp and delete them.  Note that this will
       work incorrectly if there are any filenames containing newlines or spaces.

       find /tmp -name core -type f -print0 | xargs -0 /bin/rm -f

       Find files named core in or below the directory /tmp and delete them, processing filenames
       in  such  a  way  that file or directory names containing spaces or newlines are correctly
       handled.

       find /tmp -depth -name core -type f -delete

       Find files named core in or below the directory /tmp and delete them, but more efficiently
       than  in  the  previous  example  (because we avoid the need to use fork(2) and exec(2) to
       launch rm and we don't need the extra xargs process).

       cut -d: -f1 < /etc/passwd | sort | xargs echo

       Generates a compact listing of all the users on the system.

       xargs sh -c 'emacs "$@" < /dev/tty' emacs

       Launches the minimum number of copies of Emacs needed, one after the other,  to  edit  the
       files  listed on xargs' standard input.  This example achieves the same effect as BSD's -o
       option, but in a more flexible and portable way.

EXIT STATUS

       xargs exits with the following status:
       0 if it succeeds
       123 if any invocation of the command exited with status 1-125
       124 if the command exited with status 255
       125 if the command is killed by a signal
       126 if the command cannot be run
       127 if the command is not found
       1 if some other error occurred.

       Exit codes greater than 128 are used by the shell to indicate that a program died due to a
       fatal signal.

STANDARDS CONFORMANCE

       As  of  GNU  xargs  version 4.2.9, the default behaviour of xargs is not to have a logical
       end-of-file marker.  POSIX (IEEE Std 1003.1, 2004 Edition) allows this.

       The -l and -i options appear in the 1997 version of the POSIX standard, but do not  appear
       in  the  2004  version  of  the  standard.   Therefore  you  should use -L and -I instead,
       respectively.

       The POSIX standard allows implementations to have a limit on the size of arguments to  the
       exec  functions.   This  limit  could  be  as  low as 4096 bytes including the size of the
       environment.  For scripts to be portable, they must not rely on a larger value.   However,
       I  know  of  no implementation whose actual limit is that small.  The --show-limits option
       can be used to discover the actual limits in force on the current system.

BUGS

The -L option is incompatible with the -I option, but perhaps should not be.

It is not possible for xargs to be used securely, since there will always be a time gap
between the production of the list of input files and their use in the commands that xargs
issues. If other users have access to the system, they can manipulate the filesystem
during this time window to force the action of the commands xargs runs to apply to files
that you didn't intend. For a more detailed discussion of this and related problems,
please refer to the ``Security Considerations'' chapter in the findutils Texinfo
documentation. The -execdir option of find can often be used as a more secure
alternative.

When you use the -I option, each line read from the input is buffered internally. This
means that there is an upper limit on the length of input line that xargs will accept when
used with the -I option. To work around this limitation, you can use the -s option to
increase the amount of buffer space that xargs uses, and you can also use an extra
invocation of xargs to ensure that very long lines do not occur. For example:

somecommand | xargs -s 50000 echo | xargs -I '{}' -s 100000 rm '{}'

Here, the first invocation of xargs has no input line length limit because it doesn't use
the -i option. The second invocation of xargs does have such a limit, but we have ensured
that the it never encounters a line which is longer than it can handle. This is not an
ideal solution. Instead, the -i option should not impose a line length limit, which is
why this discussion appears in the BUGS section. The problem doesn't occur with the
output of find(1) because it emits just one filename per line.

The best way to report a bug is to use the form at
http://savannah.gnu.org/bugs/?group=findutils. The reason for this is that you will then
be able to track progress in fixing the problem. Other comments about xargs(1) and about
the findutils package in general can be sent to the bug-findutils mailing list. To join
the list, send email to bug-findutils-request@gnu.org.

XARGS(1)